Field of the invention.
The invention relates generally to downhole tools for use in wells, and more particularly to means for controlling a downhole linear motor from the surface of a well in a manner that minimizes the connections that are necessary to communicate between the surface equipment and the downhole linear motor.
Related art.
In the production of oil from wells, it is often necessary to use an artificial lift system to maintain the flow of oil. The artificial lift system commonly includes an electric submersible pump (ESP) that is positioned downhole in a producing region of the well. The ESP has a motor that receives electrical signals from equipment at the surface of the well. The received signals run the motor, which in turn drives a pump to lift the oil out of the well.
ESP motors commonly use rotary designs in which a rotor is coaxially positioned within a stator and rotates within the stator. The shaft of the rotor is coupled to a pump, and drives a shaft of the pump to turn impellers within the body of the pump. The impellers force the oil through the pump and out of the well. While rotary motors are typically used, it is also possible to use a linear motor. Instead of a rotor, the linear motor has a mover that moves in a linear, reciprocating motion. The mover drives a plunger-type pump to force oil out of the well.
In order to efficiently drive a linear motor, the position of the mover within the stator must be known. Linear motors typically use three Hall-effect sensors to determine the position of the mover. These three signals are provided to a control system, which then produces a drive signal based upon the position of the mover and provides this drive signal to the motor to run the motor.
If the linear motor is to be used in a well, however, there may be a number of problems with this arrangement. For example, because the motor is positioned in a well, it is necessary to communicate the mover position signals over a substantial length (thousands, or even tens of thousands of feet) of cabling to equipment at the surface of the well. It is therefore impractical simply to provide the wires for separate electrical lines to communicate the mover position signals from the linear motor to the surface equipment. Even if the mover position signals were serially combined and communicated over a single electrical line, the higher bandwidth signal, which must be transmitted adjacent to the power cable, which carries high motor switching currents and will therefore degrade the signal-to-noise ratio of the mover position signals.
It would therefore be desirable to provide improved means for communicating necessary information about the position of the mover in a downhole linear motor to equipment at the surface of a well, and for utilizing this position information to generate signals to drive the linear motor.